Wire scribed circuit boards have become an alternative to, or may be used in conjunction with, printed circuit boards. Wire scribed circuit boards are produced by machines which bond conductors to the surface of a substrate, such as a fiberglass circuit board. The conductors are typically insulated wires of various diameters, which are applied to the surface of the circuit board in a predetermined pattern to create circuits.
The typical scribing machine has a wiring head suspended above a circuit board affixed to the surface of a table. The wiring head and circuit board move relative to each other in the horizontal x and y directions, and the vertical z-direction. Furthermore, the wiring head may rotate about the z-axis with respect to the board during turns in a conductor's path along the board.
A typical wiring head performs several functions, including guiding and feeding the conductor, bonding the conductor, and cutting the conductor. A continuous filament of the desired conductor is stored within the scribing apparatus. During processing, the conductor is despooled and fed from the wiring head onto the surface of the board. The conductor is bonded to the board by various means. For example, in one bonding technique the conductor contacts the board underneath a stylus extending down from the wiring head. The stylus transmits ultrasonic vibrations into the conductor and the circuit board activating an adhesive film on the board's surface, and causing the conductor to bond to the surface of the circuit board. Frequently, an adhesive material is applied on both the conductor and the circuit board.
Initially, the conductor follows a predetermined path as it is fed and bonded to the board. The path may include straight lines, turns, or crossing over other previously bonded conductors. At the end point, the conductor is cut. This sequence is repeated to produce the desired circuit pattern having many discrete conductor segments.
After scribing the entire circuit board, electrical connections are made to the conductor ends. Various techniques have been used. For example, holes are drilled into the circuit board at the desired locations, cutting through the insulated wire so that the exposed end of the conductor becomes part of the hole wall. The holes are then metallized, and the end of the conductor is electrically connected to the metal-plated hole.
U.S. Pat. Nos. 3,674,914 and 4,693,778 generally disclose apparatus for writing or scribing conductors onto circuit boards. Within the wiring head, a feeder mechanism feeds wire into a wire guide, which guides the wire to the surface of the board. As wire is guided beneath the tip of a stylus, the stylus exerts a downward force as it helps bond the wire to the board. A cutting blade for severing the wire is located between the end of the wire guide and the stylus.
These prior apparatus typically handled conductor wire of about 7 mils diameter. The holes receiving the conductors and for mounting the components were typically about 46 mils. With the advent of surface mounted components, the diameters of holes decreased sharply, and as a result, small wire sizes, about 2.5-4 mils in diameter, came into use. Although smaller diameter holes and wires permitted increased component and wiring densities, scribing machines were not designed to handle the smaller diameter wires and experienced problems during operation. In particular, these prior scribing machines had problems associated with their process and mechanism for cutting wires.
At the termination of a wire segment it must be cut. Such cutting is typically done by bringing a sharp blade across the wire. Initially, this cutting action was performed at the surface of the circuit board. In other words, the wire was pinched in between the blade and the surface of the board. Such a cutting technique is described in U.S. Pat. No. 3,674,914.
Cutting on the surface of the board has the advantage of being able to precisely place the end of the wire. Wire scribing systems lay wires in exact locations according to electronically stored position data. Thus, the cutting blade can be placed directly over the location on the board where the wire should be terminated. A cut, at this point, leaves the wire in a precisely known location.
The cutting mechanism, for cutting on the board surface, is important and must provide a cutting edge that cleanly and completely severs the wire. If the cut is shallow and does not extend through the diameter of the wire, the conductor could be pulled from its bonded location as the wiring head moves to the next site. Conversely, if the cut is deep, then the blade cuts into the circuit board, damaging the circuit board, dulling the blade, and possibly deforming the blade into the shape of a shovel-like lip. A dull and deformed blade not only may fail to cut wires, but also may lift the bonded wire off the board as the blade is retracted.
Cutting small diameter wires requires even greater precision. High density wire scribing results in small diameter wires being placed extremely close to each other. In order to cut only a single wire at a time, blade widths of 15 mils or less are needed, requiring precise cutting action so as not to damage the blades. Previous on board cutting mechanisms did not have to cope with small wire problems. The large blades and solenoid controlled cutting action of these prior devices provided excessive forces during cutting, making such cutting mechanisms unacceptable even for larger wire designs.
As a result of these problems, cutting techniques shifted from cutting wires on the circuit board to cutting wires above the board. The new cutting techniques prevented damage to the circuit boards and blades, but created other problems. First, it was now no longer possible to know the exact position where the wire end would be placed on the board. Second, these scribing machines could not scribe a very short segment of wire following a 90.degree. turn in the wire direction. Third, wire damage occurred when the stylus was stopped prior to reaching the wire end. Fourth, an initial feed mechanism was required.
Wire end placement error includes three sources: ultrasonic elongation, intervening crossovers, and stylus-to-cutter height variations.
Ultrasonic elongation occurs when the stylus presses down on the wire causing the wire to elongate. The elongation not only introduces wire placement error, but also increases the dc resistance of the wire and makes the wire more susceptible to damage from the hammering force of an ultrasonic stylus. Elongation can be minimized by maintaining feeder control of the wire as it is delivered from the wiring head. If the wire is cut. above the circuit board then the feeder no longer controls the wire as it passes beneath the stylus and elongation becomes a problem.
A crossover is created when a wire being scribed traverses on top of and over a wire previously scribed. When a wire is cut above the circuit board, an exactly determined amount of unbonded wire must be allowed for, in order for the wire end to precisely extend to the desired termination point on the circuit board. This amount is computed to correspond to the linear distance from the stylus tip to the end point. After wire cutting, the stylus bonds the remaining wire, ideally up to the desired end point. However, when the cut is made, their may be a crossover intervening along the path to the end point. In this case, the remaining wire not only must cover the linear distance from the stylus to the end point, but also must traverse the intervening crossover. As a result, the wire end is positioned short of the desired location.
Uncertain wire end placement also occurs when the distance between the cutter and the stylus tip varies. This distance determines the remaining length of wire to be bonded to the board's surface. Height variations in the z-direction of the circuit board cause the stylus to follow this profile. Unless the stylus and cutter are displaced together, the resulting fluctuations in the length of wire remaining to be bonded, causes the wire end to come up short or to overshoot the desired location on the circuit board.
A further problem with existing systems is their inability to produce short ends following a turn. Two factors cause this problem. First, the length of remaining unbonded wire following a cut is about 47 mils. Second, during a turn the wire has a tendency to slip out from beneath the stylus unless it has some form of directional control. Such control is normally provided by the wire guide.
After a wire is cut and has left the wire guide, directional control of the wire is lost so that no turns may be made in the wire remaining to be scribed. In existing systems this situation occurs as soon as the wire is cut, because the cutter is mounted onto the end of the wire guide. Hence, the remaining unbonded wire is positioned in a straight line and the minimum length of wire following a turn is fixed at about 47 mils.
In the current era of high density component packing, the above limitations have serious consequences. The closely spaced holes and minimum clearances required between wires and holes place a maximum length limitation on wire ends following turns. Furthermore, it is undesirable for the wire end to overshoot its intended termination point by even small amounts because the cut end may be overlapped by a subsequent wire. Then, in the presence of excessive ultrasonic action from the stylus, the wire end, which may be sharp, can damage the insulation of the top wire and result in a "short". Unnecessary, unbonded wire portions are generally problematic and should be avoided. Optimally, ends, following turns, of less than 20 mils are desired.
A further problem exists with current cutting methods. The stylus must stop about 47 mils from the desired wire end location to make the cut. The stylus then moves to bond the remaining wire. Occasionally, the stylus stops for the cut on top of a crossover. This increases the chances for wire damage.
As a result of the above described cutting method, current wiring heads must provide an initial feed mechanism. After a particular wire segment has been completely bonded to the board, there is no wire under the stylus to start the next wire. Thus, a mechanism must be provided for dispensing an initial feed of wire.
Solutions to some of these problems were attempted, such as by decreasing the cutter-to-stylus distance. Yet, these solutions were not successful and led to great mechanical complexity rendering it impractical to manufacture the wiring head.